Smartphone sales increased substantially in the second quarter of 2015, but the rate of growth continued to slow, fueling concerns that the market has started to become saturated, according to a study released today by Juniper Research.

Several versions of the Android OS for smartphones have appeared on the global stage in recent years. One version, Cyanogen OS, has gathered publicity as well as prominent investors --including Twitter, Qualcomm and Foxconn -- that could give it some staying power.